You can use only the little cubes in the first week of germination, it's easy to control the water content in the rockwool. then I transfer to the large cube when the roots are already at the bottom. At that moment the plant doesn't drink too much water, you have to be carefull because the rockwoool cube holds too much water for a seedling. You can check my recent diaries to see what i'm talking. After the transplant the plant will be good with 10-15 days without more water, only when the cube gets lighter. Some time to time the rockwool will need a reset because it will be hydrofobic (in flower I do this once per week, in veg I do this on 3rd or 4th week) fully soake the rockwool a let a 20-30% runnoff, don't do that in a single time, If you put 1 liter on a dry cube it will runnof but the cube is not fully saturated, give 20-30 minutes between small feeds (like 250ml), after 2 or 3 small feeds give them a good runnof. I think you are feeding too much water IMO, or the cube aren't holding the water (reset needed) or you are not leting dry back. Maybe the lower humidity has to do with that but in this stage one good feed should last you around 7-10 days.